1. Field of the Invention
The present invention relates to an image blur suppression device for a camera which compensates for image blur resulting from vibration or motion of an optical device of the camera, and, more particularly, to an image blur suppression device which uses an inertial pendulum system.
2. Description of the Related Art
Image blur suppression devices have as their object, suppression of, or reduction of, blurring in an image projected onto an image plane. The image blur suppression devices set forth herein are motion compensation devices. A motion compensation device is a type of image blur suppression device which compensates for motion incident upon an optical system which projects the image onto the image plane. Motion is typically imparted to the optical system by way of vibrations in the optical system, or in the surrounding holding member. In general, known motion compensation devices cause a compensation lens to shift or tilt counter to the motion of the optical system so as to shift the image projected by the optical system relative to the optical system.
FIG. 3 is a diagram of part of a conventional image blur compensation device having an inertial pendulum system. In FIG. 3, the unit indicated by slanted lines is a camera body 1. In this conventional image blur suppression device, the center of oscillation of a compensation lens group 3 and the center of oscillation of a counter balance 13 are the same. The compensation lens group 3 and the counter balance 13 are attached on opposite sides of the center of oscillation. That is, the compensation lens group 3 is attached at a position of radius L.sub.m, with point 0 as the center, and the counter balance 13 is attached to the opposite side so that the counter balance 13 is balanced with the compensation lens group 3. A spring damper system 14 comprises an elastic member 14-1, such as a spring, and a viscous member 14-2, such as a damper. The spring damper system 14 is attached at a distance L.sub.kc from the center of rotation 0.
This conventional image blur suppression device inhibits the transmission of vibrations from the camera body 1 to the compensation lens group 3 because the inertia of components such as the counter balance 13 and the compensation lens group 3 works as a low-pass filter. Motion compensation is performed by the compensation lens group 3 using the inertia to maintain the position, prior to any shifting of the camera body 1, of the compensation lens group 3.
The vibration characteristic of the compensation lens group 3, when the camera body 1 has been moved or vibrated, is obtained as the system of forced vibration resulting from displacement excitation, where:
I.sub.0 : the total moment of inertia of the suppression system PA1 .theta..sub.n : the angle of motion of the camera body 1 PA1 .theta.: the angle of motion of the compensation lens group 3 PA1 (1) Increase the moment of inertia of the compensation lens group 3 PA1 (2) Decrease the spring constant of the elastic member
When the camera body 1 is considered to vibrate at .theta..sub.n =.theta..sub.0 sin (.omega.t), the equation of motion becomes: EQU I.sub.0 d.sup.2 .theta./dt.sup.2 +cL.sub.kc.sup.2 d.theta./dt+kL.sub.kc.sup.2 .theta.=cL.sub.kc.sup.2 d.theta..sub.n /dt+kL.sub.kc.sup.2 .theta..sub.n
Also, the intrinsic angular vibration frequency of this vibration system is : EQU .omega..sub.n =L.sub.kc (k/I.sub.0).sup. 1/2
FIG. 4 is a graph of this convention image blur suppression device which has undergone forced vibration. The vertical axis of FIG. 4 shows the vibration transmission rate M to the compensation lens group 3 when the camera body 1 has been vibrated. The value of the amplitude of vibrations of the compensation lens group 3 divided by the amplitude of vibrations to the camera body 1 is shown by [dB]. The horizontal axis of FIG. 4 is the vibration frequency ratio, which is the ratio (.omega./.omega..sub.n) of the angular vibration frequency of the camera body 1 with respect to the intrinsic angular vibration frequency (.omega..sub.n) of the conventional image blur suppression device. Camera motion during hand-held photography is mostly due to vibrations within a frequency band of approximately 1[Hz].about.15[Hz]. When the value of the transmission rate M is not more than -20[dB], there is little displacement of the compensation lens group 3 due to vibration of the camera body 1, and the image blur suppression device of FIG. 3 performs effective image vibration compensation.
As can be seen in FIG. 4, because the image blur suppression device deals with motion in the low frequency range of the camera, it is necessary to set the fixed frequency of the convention image blur suppression device to a sufficiently low value. To lower the fixed frequency, the following may be done:
In the first case, as (moment of inertia)=(mass).times.(radius of gyration).sup.2, it is advantageous to lengthen the effective radius of gyration.
In the conventional image blur suppression device, to deal with camera motion in the low frequency range, the fixed vibration frequency of the compensation lens group 3 may be lowered, and the moment of inertia of the compensation lens group 3 may be increased. However, to do this, it is necessary to lengthen the radius of oscillation of the compensation lens group 3, which would increase the length of the camera in the optical axis direction. Moreover, even where the radius of oscillation of the compensation lens group 3 is the same, the length, in the optical axis direction, can not be shortened by bringing the center of oscillation closer to the film plane.
Also, the amount of de-centering from the photographic optical axis of the compensation lens group 3 in the conventional image blur suppression device is determined from the positions of the center of rotation of the camera body 1 and the center of oscillation of the compensation lens group 3 to balance the counter balance 13, thereby limiting freedom of design.